1. Field of the Invention
This invention relates to mass spectrometry systems and methods, and more particularly to mass spectrometry for detecting the secondary ions emitted by a focused ion beam striking a target.
2. Description of the Related Art
Typical secondary ion mass spectrometry (SIMS) systems operate with a relatively large ion beam diameter of about 10-100 microns. However, several applications have been identified for a SIMS system used in connection with a focused ion beam (FIB) having a small focused diameter of about 0.02-0.2 microns. These include localized impurity and compositional measurements in small areas of integrated circuits, the localized depth profiling of layered structures, and high resolution SIMS imaging of integrated circuit contamination and underlying structures. The FIB/SIMS technique has considerable potential for evaluating and understanding both the fabrication processes and the resultant circuit structures in state-of-the-art integrated circuits. Also, the application of FIB/SIMS to contamination identification in the integrated circuit fabrication process is quite valuable when the entire sample volume is very small, such as would be the case when the sample is a contaminant of unknown origin with submicrometer dimensions.
A typical large beam SIMS system is described in a set of technical notes published by Cameca, "IMS 3F Secondary Ion Mass Spectrometer", while a FIB/SIMS system is described in an article by R. Levi-Setti, G. Crow and Y. L. Wang, "Progress in High Resolution Scanning Ion Microscopy and Secondary Ion Mass Spectrometry Imaging Microanalysis", Scanning Electron Microscopy, Vol. 2, 1985, pages 535-51. Conventional ion mass spectrometers analyze the mass/charge ratios in a charged particular beam by accelerating the charged particles through a fixed electric field, and then deflecting them by means of a variable magnetic field onto to a fixed detector. As the magnetic field is varied, particles of varying mass are scanned across the detector. Quadrupole systems have also been used for mass separation, with the mass selection being determined by the voltage amplitude applied to the quadrupole. In either case, particles of only one mass are detected at any given instant of time, and all other particles are discarded. Thus, conventional ion mass spectrometry is analogous to a serial system. If the system has a range of 200 atomic mass units (AMUs) and a resolution of 0.2 AMU, for example, on the average only 1/1000th of the available particles are detected at any one time. This makes the systems quite inefficient, and transistory phenomenon associated with one or more particular ion masses may be missed altogether if those masses are not sampled at the right time.
An increase in efficiency is especially important for FIB/SIMS applications in which high resolution is obtained but at reduced primary ion beam currents. Typical beam sizes for FIB/SIMS are generally about 500-2,000 angstroms, but corresponding incident beam currents at the target are usually about 25-100 pAmps, or roughly 1000 times less than the current in conventional SIMS systems. Thus, long integration times are necessary to obtain a good signal-to-noise ratio.
This inefficiency is particularly critical for analyzing the composition of small contaminants of unknown origin on a semiconductor integrated circuit wafer with FIB/SIMS, since the total number of available secondary ions is limited. Also transit time considerations of a quadrupole impose further constraints on the analysis of small contaminants. The speed at which a mass spectrum can be obtained is limited by the requirement that the RF quadrupole voltage must be roughly constant during the transit period of the ions in each resolution segment. However, the rate of erosion of the contaminant being analyzed is a function of the current in the primary beam. When submicrometer regions are being analyzed, only a fraction of a complete spectrum can be made before the region is consumed by the beam. Thus, the analysis of regions less than 1/2 micrometer in size with a 1000 angstrom diameter FIB using serial ion mass spectrometry is extremely difficult, if not impossible.
One of the unique features of FIB/SIMS systems is the ability to obtain high resolution elemental maps in both 2 and 3 dimensions. However, presently available mass scanning techniques require many hours of analysis time if a mass scan is made during each beam dwell period. Alternatively, if only one ion mass is observed per single raster scan, the time required to observe all masses (assuming a dwell time per pixel sufficient to obtain good picture quality) is again in the order of 75 hours. Moreover, since a deep sputtered crater is formed during hours of analysis, surface information is lost with the prior ion mass spectrometry approach.